An emitter-switching circuit configuration comprises a bipolar transistor having a high breakdown voltage connected to a low voltage power MOSFET transistor. Such a configuration is schematically shown in FIG. 1 and is indicated with reference numeral 1. The emitter-switching configuration 1 comprises a bipolar transistor T1 and a MOS transistor M1 cascode-connected together between a load 3 and a voltage reference, such as ground GND.
The emitter-switching configuration 1 provides that the bipolar transistor T1 is of the high voltage (HV) type, i.e., a high breakdown voltage transistor, while the MOS transistor M1 is of the low voltage (LV) type, i.e., a low breakdown voltage transistor. The bipolar transistor T1 has a collector terminal connected to the load 3, and a control terminal or base B1 connected to a driving circuit 2.
The load 3 is of the resonant or quasi-resonant type and comprises an inductor L1 corresponding to the primary of a transformer, and a capacitor C1 inserted in parallel with the inductor L1. The capacitor C1 is between the collector terminal of the bipolar transistor T1 and a supply circuit node X1. The supply node X1 is connected to a generator GB that provides a supply voltage Vcc, which is applied to the supply circuit node X1.
The capacitor C1 is chosen so that it resonates with the inductor L1 based upon the operating frequency of the intended application. The MOS transistor M1 has a control terminal or gate connected to the driving circuit 2.
The driving circuit 2 comprises a first resistive element R1 connected to the gate terminal G1 of the MOS transistor M1, and to ground GND via a voltage pulse generator GA. An electrolytic capacitor C2 is connected between the base terminal B1 of the bipolar transistor T1 and ground GND, and has across its terminals a voltage value equal to VB. A second inductor L2 corresponding to the secondary of a transformer is inserted between a second circuit node X2 and ground GND. A diode D1 is inserted between the base terminal B1 of the bipolar transistor T1 and the second circuit node X2.
The emitter-switching configuration is particularly interesting at the present time due to the marketing of bipolar transistors having a square RBSOA (Reverse Biased Safe Operating Area) with a current near the peak current. It also has a voltage equal to the breakdown voltage BVCES between the collector and emitter terminals when the base terminal is short-circuited with the emitter terminal (Breakdown Voltage Collector-Emitter Short), as well as of MOS power transistors having a very low drain-source resistance value in conduction conditions RDSON and thus being almost similar to ideal switches.
The main advantages of an emitter-switching configuration are an extremely low in-conduction voltage fall (typical of bipolar transistors) and a high turn-off speed, as readily known by those skilled in the art. When turning off, the current output from the bipolar transistor base terminal B1 is equal to the collector terminal current of this transistor, i.e., a very high value current. This causes a drastic reduction in both the storage time and the fall time, allowing the emitter-switching configuration to operate even at frequencies of 150 kHz.
All the applications satisfying the following relation will now be considered:IBOFF*tstorage>>IBON*tONWhere:
IBOFF is the base current value of the bipolar transistor T1 in the turn-off step;
tstorage is the storage time;
IBON is the base current value of the bipolar transistor T1 in the conduction step; and
Ton is the conduction time.
The above relation occurs when the operating frequency is relatively low (i.e., lower than 60 kHz), and particularly if high currents (higher than ten Amperes) are being dealt with. This is the case for rice cooker devices, for example, whose waveforms are indicated by way of example in FIG. 2 for an operating frequency of 35 kHz and with a highest collector current of about 40 A. In this case, the driving circuit of FIG. 1 is straightforward but expensive. In fact, it is necessary to provide a middle power supply (about 10 W).
Only the secondary part of the relevant power supply has been indicated in FIG. 1 for convenience of illustration. The power supply is essential since the base of the bipolar transistor T1 should be supplied with a current IB, which in the present application and in all similar applications, cannot be provided by recovering energy during the turn-off, as it happens instead in other applications.
For a better understanding of this concept it must be observed in FIG. 2 that the area A2, representing the amount of charge recovered during the turn-off, is far lower than the area A1 representing instead the amount of charge required by the base terminal B1 to make the switching configuration 1 operate correctly. This known approach is thus effective from a performance point of view, but is very expensive. Other known approaches provide for the use of IGBT or MOS power transistors.
When the following conditions occur: resonant or quasi-resonant load; high collector current (>10 A); relatively low frequency (<60 kHz); and not too low a duty cycle (>15%), the devices formed by IGBT power transistors are particularly suitable. In fact, in view of voltage levels, current values and the relatively low frequency, choosing a MOS power transistor (PowerMOS) would be particularly expensive.
A MOS power transistor able supporting voltages higher than 1000 V and conducting a high current would involve a silicon area from 5 to 10 times higher than an IGBT transistor or a bipolar transistor having the same breakdown voltage value and current capacity value. Moreover, a MOS power transistor would be unnecessary since its switching speed thereof, which is instead essential at higher frequencies, would not be fully exploited.
A bipolar transistor in the emitter-switching configuration has highly competitive turn-off times, and in particular, fall times comparable to those obtained with a MOS power transistor. It also has a very low in-conduction fall, the lowest obtainable with power devices, as illustrated in FIG. 4.
By using an IGBT transistor, a relatively low in-conduction voltage fall is similarly obtained, as indicated by the output features shown in FIG. 3. However, the IGBT transistor has a current queue when turning off, a phenomenon which can be completely deleted by using a bipolar transistor in the emitter-switching configuration (by conveniently driving the base terminal), as shown in FIG. 2. The use of a bipolar transistor in the emitter-switching configuration is thus limited with respect to the base terminal driving current.